The present invention concerns the field of dental mechanics and more particularly the recording of impressions of the dental arches, which constitutes the necessary prerequisite for subsequent construction of a movable prosthesis.
The present invention covers a new composition with which a true mucodynamic impression of the dental arches can be obtained and which exhibits chemical and physical characteristics that are entirely new in comparison with the conventional materials.
The impression of a patient's dental arches is at present recorded with various types of materials. A first group of such materials, well known in the dentistry field, consists, for example, of alginates, irreversible colloids, plaster of Paris, zinc oxide and eugenol paste, silicones, and polyether resins. With these materials, however, the osteomucous complex of the patient's mouth is fixed in a static position (patient motionless with mouth open), and the impression obtained--a so-called "mucostatic" impression, relates to the condition in which the tissues are placed at the precise moment of recording, whereas it is well known that during mastication the soft tissues on which the prosthesis will rest undergo numberless shifts and movements, which cannot be exactly defined with any apparatus, and even less can they be recorded by means of one or more impressions. The proof of this, as all dentists are well aware, can be seen in the fact that if a score of impressions are taken from the same patient with the same material, no two that are identical with each other will be found at the end.
It follows that a dynamic impression is necessary, i.e. an impression with characteristics such as to be faithful to all movements the patient makes in talking, smiling, chewing, etc.
For this purpose the so-called "conditioning" resins, which are in fact only normal acrylic resins containing plasticizers to keep them soft, have been brought onto the market.
If they are applied at the base of the prosthesis and if the patient is made to carry out certain mandibular movements, these resins reproduce the movement of the soft tissues with a certain approximation; after a few minutes, however, as they are thermosetting and not thermoplastic, they no longer change their form. Left in the mouth for a period ranging from a week to several weeks, these resins act on the oral tissues and "condition" them, i.e. modify the position and mobility of the tissues; when this has been achieved, they are removed and replaced by rigid acrylic resin forming part of the prosthesis.
It is thus evident that even in this case the impressions are not mucodynamic, but we have an "adjustment of the tissues to the impression" and for this reason they are known as conditioning resins.
To record a true mucodynamic impression that is the sum or resultant of the multiple tissue variations that take place over time, it is thus necessary to find a material with entirely different functional characteristics, and in particular which can:
(a) be spread on the base of the dental plate and modelled on the dental arches, PA0 (b) retain its plasticity for a sufficient period of time to reproduce all those above-mentioned numberless spontaneous movements of the tissues, and PA0 (c) harden enough to be subjected to the subsequent processing stages.